In a magnetic disk device such as a hard disk drive (HDD), a recording/reproducing head that floats above a rotating magnetic disk records data onto the magnetic disk and reads data from the magnetic disk.
It is known that the recording density of a magnetic disk in a HDD can be increased by narrowing the space (the distance) between the recording/reproducing head and the magnetic disk (see Japanese Patent Application Publication No. 2005-235380, for example). As the distance between the recording/reproducing head and the magnetic disk becomes shorter, however, there is a higher possibility that the data recorded in the tracks adjacent to a predetermined target track is overwritten due to the magnetic field generated when data is written onto the target track. This is also called “write spread”, and due to the write spread, the data recorded in the adjacent tracks might be erased (a so-called “side erase”).
To counter this problem, the HDD manufacturers conduct tests to determine the number of write times that does not cause write spreads, and use the technique for guaranteeing the number of write times. In one of those tests, data is repeatedly written onto the target track the guaranteed number of write times that is set for each HDD model, and data is read from the tracks adjacent to the target track. The degree of degradation of the signal quality is then measured. The guaranteed number of write times set in this test may be 100 millions, for example.
In a case where data is read from the adjacent tracks after data is repeatedly written onto the target data the guaranteed number of write times, as described above, the number of times the data is actually written is enormous, and a long period of time might be required between the start and the end of each test. To shorten the test time, the degree of degradation may be measured after data is written onto the target track only 1/10 or 1/100 of the guaranteed number of write times, and the degree of degradation corresponding to the guaranteed number of write times may be estimated from the measured degree of degradation.
In the case where the degree of degradation corresponding to the guaranteed number of write times is estimated, however, there is a large difference between the estimated degree of degradation and the degree of degradation measured after the data is actually written the guaranteed number of write times. Therefore, there is a high probability that the degree of degradation cannot be accurately estimated. The inventor conducted various examinations on the cause of this problem, and has reached the conclusion that the problem lies in the method of measuring the degree of degradation.